Put alcohol on rag and touch to the finish in place where it will not be seen usually on the rear arm where the motor boss would be. If the finish gets tacky or soft then it is shellac. If nothing happens then it is a varnish of somekind and this tech will not work. You can spray on the clear shellac or you can use finger and rag. Did finger in a little shellac with a couple of drops of linseed oil and lightly rub on the finish in a circular motion then lift your finger and go over lightly in one direction.
Most machines 60 years + will normally be shellac. This shellac clear coat was applied to protect the decals and make the japan black shine a little. Hope this helpr you. Skip